Annealing temperature dependent structural and optical properties of electrodeposited CdSe thin films

Cadmium selenide films were synthesized using simple electrodeposition method on indium tin oxide coated glass substrates. The synthesized films were post annealed at 200 °C, 300 °C and 400 °C. X-ray diffraction of the films showed the hexagonal structure with crystallite size <3 nm for as deposited films and 3–25 nm for annealed films. The surface morphology of films using field emission scanning electron microscopy showed granular surface. The high resolution transmission electron microscopy of a crystallite of the film revealed lattice fringes which measured lattice spacing of 3.13 Å corresponding to (002) plane, indicating the lattice contraction effect, due to small size of CdSe nanocrystallite. The calculation of optical band gap using UV–visible absorption spectrum showed strong red-shift with increase in crystallite size, indicating to the charge confinement in CdSe nanocrystallite.     

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10 nm for ALD processed Al₂O₃. In this work, we investigate the barrier performance of Al₂O₃ films processed by ALD at 80 °C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5 nm thick Al₂O₃ film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20 × 20 μm² scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100 nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25 nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25 nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al₂O₃ permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25 nm.     

Effect of organic dopants on electrodeposition and characteristics of polyaniline under the varying influence of H2SO4 and HClO4 electrolyte media

Electrodeposition of conducting polyaniline (Pani) was made under potentiostatic condition at pH 1.0 in different electrolyte media (H₂SO₄ and HClO₄) in the absence and presence of two organic dopants, disodium salts of naphthalene-1,5-disulphonic acid (NSA) and of catechol-3,5-disulphonic acid (CSA). The rate and yield of Pani deposition were dependent on the acid medium and the dopant employed. NSA in H₂SO₄ caused an increase in rate and yield but CSA decrease when compared to the rate and yield of H₂SO₄ alone. In HClO₄ medium, both the dopants showed a decrease. With regard to DC electrical conductivity, both the dopants exhibited an enhancement in H₂SO₄ medium but NSA a decline in HClO₄. Characterisation of the electrosynthesised polymer samples by various instrumental techniques (cyclic voltammetric: CV, FTIR, UV-Visible: UV-Vis, EPR, XRD, TGA and DTG methods) revealed that between the two acid media, H₂SO₄ was the better one. Further, it enlightened the role of two organic dopants in relation to the acid media. The advantageous role of NSA in H₂SO₄ had origin on its molecular characteristics such as non-polarity, larger π-electron cloud etc., while CSA could not perform such a role because of its easily oxidisable hydroxyl groups. In HClO₄, however, both the dopants could play only an unfavourable role owing to its greater polarity and oxidizing power than H₂SO₄.     

Non-aqueous electrodeposition of ZnO and CdO films

ZnO films were electrodeposited from a dimethylsulfoxide bath containing dissolved gaseous oxygen. Variations in deposition parameters and their effects on the structural (crystal size, growth direction), optical (bandgap variations, photoluminescence) and electrical (conductivity) properties are described. The technique was extended to give highly-conducting films of CdO.     

The growth of roughness during electrodeposition

Measurements of the growth rate of roughness on a cathode surface can be used to study the physics of cathodic processes. But these growth rates are cell wide variables and they ordinarily depend on what is taking place at the anode as well as at the cathode. Our aim is to derive a simple condition under which the anode makes no contribution to the interpretation of growth rate measurements at the cathode.Our condition stems from the contribution of ion diffusion to the growth rate and it is satisfied for all non-small values of the wave number of a growing disturbance. This, together with the fact that surface tension is the only stabilizing factor and the fact that the surface tension coefficient multiplying the wave number is small, means that the anode ought not to be important for the fastest growing wave numbers.Once the effect of the anode is eliminated, the growth rate at the cathode can be expressed in a formula so simple that pencil and paper calculations are possible and this makes important questions in cell design easy to answer, e.g., is the growth rate diffusion controlled?     

Oxidation of some aliphatic polyols on anodically deposited MnO2

Galvanostatic steady state current potential measurements were carried out for oxidation of a series of aliphatic alcohols having varying number of hydroxyl groups. The anodically deposited layer of MnO₂ on platinum was used as the electrode material. The deposit was characterised by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrode potential measurements. The catalytic role of MnO₂ in the electro-oxidation of alcohols was indicated by the chronopotentiograms and the cyclic voltammograms. An analysis of the electrochemical data indicated a catalytic EC mechanism in which Mn (V) is generated electrochemically and consumed chemically in succession. Based on this and the hydrogen bonding interaction between alcoholic hydroxyl groups and MnO₂ layer, a mechanism was proposed which accounts for the variation in the observed electrochemical reaction orders. Tafel behaviour was found to be followed only approximately. Current efficiency of the electrochemical oxidation of polyols was studied. Replacement of platinum by carbon as current collector was found to leave the electrocatalytic activity of the MnO₂ deposit practically unaltered.     

Preparation of the TiB2 coatings by electroplating in molten salts

The improvement of titanium diboride (TiB₂) coatings over the molybdenum substrate was investigated by comparing the electrochemical techniques of continuous current plating (CCP) and periodically interrupted current (PIC). The solvent used was a eutectic Flinak mixture (LiF-NaF-KF, 46.5:11.5:42 molar ratio) with solutes K₂TiF₆ and KBF₄ with the electrochemically-active components in the molar ratio of one to five. The coatings produced by PIC show improvements in morphology and microstructure for the suitable conditions: i = 0.5 A/cm², frequency = 100 Hz, the time ratio t_c/t_off (current on/current off) = 4/1. X-ray diffraction (XRD) analyses indicated that the coatings are composed of the relatively pure TiB₂ and the preferred orientation is [001] + [110], which is in accordance with the prediction of the two-dimensional crystal nuclei theory.     

Silver deposition on silicon and glassy carbon. A comparative study in cyanide medium

Nucleation and growth during Ag deposition on n-Si (111) have been studied and compared with results obtained on glassy carbon (GC). The initial stages of metal deposition from a cyanide electrolyte (pH 14) were investigated using conventional electrochemical techniques combined with AFM and SEM. Relevant kinetic parameters were obtained from the analysis of current transients on the basis of existing models for electrochemical nucleation and diffusion controlled growth. On n-Si a clear change in the Ag deposition mechanism from progressive (E=−0.80 V vs. standard hydrogen electrode, SHE) to instantaneous (E=−0.90 V vs. SHE) nucleation is observed, while on GC an intermediate behaviour is found at both potentials. A strong dependence of the nucleation site density N₀ with potential E was observed on both Si and GC, being this effect particularly important in the case of the semiconductor. Therefore, the driving force for Ag nucleation (supersaturation) on Si was varied both by changing the electrode potential E at c_AgCN=constant and the concentration c_AgCN at E=constant. On silicon, the critical silver nucleus was found to be composed by one atom within the studied potential range. The stability of the Ag clusters in the cyanide electrolyte was found to be strongly influenced by the presence of oxygen. Studies of the solid state n-Si/Ag contact indicate an ideal Schottky behaviour and the formation of a high quality junction.